The bactericidal/permeability increasing protein (BPI) is a potent antimicrobial agent of 456 residues. BPI resides in the lysosomal granules of polymorphonuclear neutrophils, which are mobilized by the host to kill invading microorganisms. Recently, the structure of BPI was solved to 2.4 E resolution in our laboratory. We wish to continue structural studies on this portein to further characterize the structure to higher resolution, as well as determine cocrystal structures of BPI with several physiologically relevant ligands that will help elucidate its function.